The present invention relates to a method and a means for feeding electric energy to a portable power tool.
In particular, the invention concerns a method and a means for feeding energy to a portable power tool comprising a brushless electric motor.
The primary object of the invention is to accomplish an improved power-weight relationship as well as a stiffer power-speed relationship in electrically powered tools. This is achieved by automatically adapting the parameters of the supplied electric energy to the instantaneous operating conditions of the tool motor in relation to predetermined performance specifications.
Prior to this invention there have been available mainly two different types of electric tools as far as the electrically powered prime mover is concerned. One of these comprises a brush fed motor for connection to a conventional mains voltage of 50 or 60 Hz frequency. The other type comprises a brushless motor for connection to a non-variable voltage source of 150-400 Hz non-variable frequency.
The first mentioned type of tool including a brush fed mains connected motor is disadvantageous in that the brushes produce sparks which may be hazardous in inflammable or explosive atmosphere, and in that the brushes and commutator are exposed to a hard wear, especially when the tool is used in a dusty and/or corrosive atmosphere.
Concerning the operational features, the brush motor tools have by nature a high idle speed and a very weak power-speed characteristic. Lately however, tools of this type have been equipped with electronic control means by which the idle speed is reduced to a suitable level. At the same time a stronger motor has been employed such that an increased output power is obtained up to this new lower idle speed level. This means that the power-speed characteristic is improved and that the motor speed is better kept up at increasing load on the motor. Yet, the power-speed relationship is not stiff enough to satisfy heavy duty demands. In, for instance, portable grinding machine applications a loss of speed at increasing load causes a rapidly increasing wear of the grinding tool.
The brush fed motor tools are disadvantageous also in that they have a low output power-weight relationship. This means that for a given output power the weight of the tool is rather high, which of course is a serious drawback for a portable tool.
The brushless high frequency motor tools have a stiffer power-speed characteristic than the brush fed motor type tool, which means that the brushless tool better keeps up the speed at increasing motor load. The stiffer the power-speed relationship is the better from the viewpoint of tool wear as well as production rate.
Previously available brushless motor tools, however, are disadvantageous in that their power feed means provide electric energy of constant voltage and frequency. This means that due to the lack of correlation between motor load and voltage amplitude over large parts of the speed range, the energy losses are high and overheating of the motor is a serious performance limiting factor for this type of tool.
Previous brushless motor tools as disadvantageous also in that their use is limited to such areas where the non-portable high-frequency power feed means is within reach. The type of power feed means previously used in connection with portable tools is of the rotary converter type.
Such converters are not only limited to specific and predetermined voltages and frequences but have by nature a high weight-to-power ratio and are not possible to make portable. For that reason, prior art power supply means of this type are arranged to serve a number of tools through a local network, confined for instance within the premises of a factory.
The above described problems concerned with prior art technique in this field are solved by the invention as defined in the claims.